Catheter

ABSTRACT

In the present invention, a catheter comprises a resiliently biased projection and a detector which generates a signal which varies as a function of radial displacement of the resiliently biased projection relative to the longitudinal axis of the catheter. In a preferred example, the catheter comprises a resiliently biased projection comprising one plate of a variable capacitor, wherein the capacitance varies as a function of radial displacement of the resiliently biased projection. Thus, a signal processing system electrically coupled to the variable capacitor plate may be adapted to detect changes in the capacitance of the variable capacitor. This affords a method of studying the physiology and/or morphology of a vessel wall by detecting capacitance variations between capacitor plates inserted into the vascular tissue. The present invention allows dimensional characteristics of the vascular tissue to be determined. For example, the cross-section of a vascular lumen can be measured by measuring the capacitance between the plates of the variable capacitor and relating this to the distance between the plates. If the position of one of the plates in the lumen is known, for example, being positioned against one wall of the vessel, the resiliently biased projection, comprising the other plate of the variable capacitor, can bias itself against the opposing wall. The measured capacitance is directly proportional to the distance between the plates.

BACKGROUND TO THE INVENTION

The human vascular system may suffer from a number of problems. Thesemay broadly be characterised as cardiovascular and peripheral vasculardisease. Among the types of disease, atherosclerosis is a particularproblem. Atherosclerotic plaque can develop in a patient'scardiovascular system. The plaque can be quite extensive and occlude asubstantial length of the vessel. Additionally, the plaque may beinflamed and unstable, such plaque being subject to rupture, erosion orulceration which can cause the patient to experience a myocardialinfarction, thrombosis or other traumatic and unwanted effects.

The study of the vascular wall has proven to be of incomparable valuefor the percutaneous study for the majority of cardiac diseases. Severaltechniques have been developed for studying vascular tissue. However,existing methods based on intravascular ultrasound give limitedmorphological information concerning the tissue characterisation of thearterial wall. Other methods include the measurement of variousparameters such as blood pressure, flow velocity, temperature, impedanceand the like. These techniques provide poor, or no information about thecomposition of the vascular tissue. In particular, the above techniquesdo not provide selective information about the different tissues whichmake up the vascular wall.

There is a need to produce a method which can be used to detect thecomposition of the vascular tissue and to provide anatomical andmorphological data, thereby yielding information about the quality ofthe vascular tissue. Analysis of the vascular wall composition can beused to detect early atherosclerosis and other diseases and adverseconditions affecting the vascular tissue, thus rendering the possibilityof early treatment of the condition. This allows the possibility ofprevention, rather than just cure of such conditions.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, a cathetercomprises a resiliently biased projection and a detector which generatesa signal which varies as a function of radial displacement of theresiliently biased projection relative to the longitudinal axis of thecatheter.

According to a second aspect of the present invention, a catheter systemcomprises a catheter in accordance with the first aspect of the presentinvention, in combination with a signal processing system electricallycoupled to the detector, which is adapted to detect changes in thesignal of the detector.

According to a third aspect of the present invention, a method ofstudying the physiology and/or morphology of a vessel wall is providedby detecting signal variations in detectors inserted into the vasculartissue.

A preferred example of the first aspect of the present inventionprovides a catheter comprising a resiliently biased projectioncomprising one plate of a variable capacitor, wherein the capacitancevaries as a function of radial displacement of the resiliently biasedprojection. Thus, a signal processing system electrically coupled to thevariable capacitor plate may be adapted to detect changes in thecapacitance of the variable capacitor. This affords a method of studyingthe physiology and/or morphology of a vessel wall by detectingcapacitance variations between capacitor plates inserted into thevascular tissue.

An alternative example of the first aspect of the present inventionprovides a catheter comprising an inductance coil and a magnet. Eitherthe inductance coil or the magnet may be mounted on or be integrallyformed with the resiliently biased projection, wherein the inductance inthe coil varies as a function of radial displacement of the resilientlybiased projection. Thus, a signal processing system electrically coupledto the inductance coil may be adapted to detect changes in theinductance of the coil.

The present invention allows dimensional characteristics of the vasculartissue to be determined. For example, the cross-section of a vascularlumen can be measured by measuring the capacitance between the plates ofthe variable capacitor and relating this to the distance between theplates. If the position of one of the plates in the lumen is known, forexample, being positioned against one wall of the vessel, theresiliently biased projection, comprising the other plate of thevariable capacitor, can bias itself against the opposing wall. Themeasured capacitance is directly proportional to the distance betweenthe plates.

One of the advantages of the present invention is that a direct contactmethod of evaluating the vascular dimensions may be employed. Inparticular, this has the advantage of employing other types of directmeasurement, for example, physiological parameters such as temperatureof the vascular tissue, and integrate these with the dimensionalcharacteristics of the vascular tissue. This can provide an enhanced“picture” of a portion of vascular tissue. For example, it has beenreported that unstable and inflamed plaque can cause the temperature ofthe artery wall to elevate up to 2.5° C. The present device and methodallow the user to establish that not only an inflammation of the vesselis present, possibly indicating stenosis of the vascular tissue, butalso that the temperature in the same region is elevated compared tosurrounding, non-inflamed tissue. This allows the identity andprioritisation of treatment for unstable plaque.

Where the detector is a variable capacitor, at least one of the platesare attached to the resiliently biased projection or form an integralpart thereof. Preferably, pairs of capacitor plates are attached to, orare integrally formed with pairs of opposing resiliently biasedprojections. Most preferably, at least one plate of the variablecapacitor is formed integrally with the resiliently biased projection.Preferably, at least a portion of the variable capacitor forms asubstantially flat surface. The variable capacitor is preferablyconstructed from metal plate, metal foil or a metal film deposited on asubstrate. Preferred metals include nickel, titanium, gold, steel,silver and alloys thereof. The electrolytic capacitance created betweenthe plates uses the blood as dielectric. Blood has a high dielectricconstant e (serum plasma has approx. e=200, measured at 1 MHz). Thus,with a plate separation of 5 mm and a plate area of 1 mm² each,capacitance should be C=8.85×10⁻¹²×200×0.000001/0.005=0.354×10⁻¹²Farads. At a plate separation of 1 mm,C=8.85×10⁻¹²×200×0.000001/0.001=1.77×10⁻¹² Farads.

Where a variable capacitor is used as the detector, the physiologyand/or morphology of a vessel wall is investigated preferably using thefrequency shifting of an oscillator due to capacitance variations of acapacitor formed between plates carried by a catheter. Changes incapacitance presented by this variable capacitor are detected by thefrequency shifting of an associated variable frequency oscillator whichis mixed with the output of a fixed frequency oscillator.

Preferably, the signal processing system according to the second aspect,comprises one or more variable frequency oscillators, the outputfrequency of which is frequency shifted in dependence on the capacitancepresented by a respective variable capacitor of the catheter.

In a preferred example, the signal processing system comprises a firstoscillator, the output frequency of which is dependent on thecapacitance presented by a respective variable capacitor of thecatheter, and a second oscillator, the output frequency of which isfixed, and a frequency mixer which receives the signal outputs of thefirst and second oscillators and generates a difference frequencysignal.

Preferably, the variations in capacitance are detected by the frequencyshifting of an oscillator.

The catheter of the present invention is particularly useful forintravascular studies, but equally can be used in other organs orcavities for studying their morphological characteristics and wallcomposition. Sophisticated computer processing of data can provideinformation on vascular wall composition and morphology that is hithertounavailable.

Where the detector comprises an inductance coil, it may be attached to,or integral with the resiliently biased projection. Alternatively, theinductance coil may be mounted on, or formed integrally with the body ofthe catheter. The coil is preferably mounted so that it lies onsubstantially the same axis as the magnet. The coil should preferablyhave a relatively flat profile coil. The coil preferably consists of1-200 loops. The coil diameter is preferably in the range of 0.25-5 mm,more preferably 0.5-2 mm, most preferably about 1 mm. The coil length(measured along its axis) is preferably in the range of 0.1-10 mm, morepreferably 0.5-5 mm, most preferably about 1 mm. The coil is preferablyconstructed from a metal. Suitable metals include silver, gold, nickel,copper and alloys thereof.

The magnet may be attached to, or integral with the resiliently biasedprojection. Alternatively, the magnet may be mounted on, or formedintegrally with the body of the catheter. Alternatively, the catheterguide-wire may be magnetised in order to provide the function of themagnet The magnet may be constructed from any suitable magneticmaterial. The magnet is preferably constructed from AlNiCo alloy,Ceramics, samarium cobalt alloy, Neodymium Iron alloy, Iron-chromealloy, and the like. In a particularly preferred embodiment, the coil ismounted on the projection or the body of the catheter, and a secondprojection is constructed, at least in part from the magnetic material.

Where the detector comprises an inductance coil, one of either theinductance coil or the magnet should be mounted on the resilientlybiased projection.

A particular advantage of using variable capacitors or inductance coilsis that these methods are particularly sensitive and high resolution ofthe vessel walls can be obtained. Angstrom changes in distance canresult in changes in frequency in the order of KHz.

Such highly sensitive techniques enable the device of the presentinvention to distinguish between the systolic and diastolic diameters ofthe blood vessel. Consequently, this enables the user to measureparameters such as the elastic index of the blood vessels. This isparticularly useful as it provides information about the physiology ofthe vessels being studied. For example, a stenosed or calcified regionof blood vessel is generally less elastic than a healthy region.

One or more detectors, preferably 2 to 10 detectors, more preferably 2to 6 detectors may be utilised in the present invention. Preferably,each detector is mounted on a separate projection. In a particularlypreferred example, four projections, each having a single detectormounted thereon, are provided.

Generally, the catheter of the present invention comprises a pluralityof co-axial lumen. Preferably, the catheter comprises a central lumenadapted to be mounted on a standard angioplasty guide wire suitable forvascular intervention. The apparatus is preferably based on therapid-exchange or the monorail system, although over-the-wire techniquesare also envisaged. Preferably, outside the central lumen is located anintermediate lumen. Preferably, outside the intermediate lumen ismounted an external lumen, hereinafter referred to as a sheath.Preferably, at the distal tip of the apparatus is a guide member. Otherlumen may be present and all the lumen may house components withinthemselves or between adjacent lumen.

The projection is preferably mounted on the central or intermediatelumen but may be attached to any lumen inside the sheath.

The central lumen may be formed from the standard catheter lumenmaterials, for example, nylon, FEP, polyurethane, polyethylene andnitinol and mixtures thereof.

The intermediate lumen and the sheath are generally constructed from,but individually selected from, the standard catheter lumen materialsdiscussed above.

The sheath is adapted to fit over the adjacent lumen housed inside thesheath and should be able to move relative to the adjacent lumen underthe control of a remote device.

Preferably, the central and intermediate lumen are bound to one anotherand are not moveable relative to one another.

Preferably, the flexible body of the catheter has a longitudinal axisand at least part of the projections are extensible radially from thelongitudinal axis of the body. Generally, the projections have anelongate shape, preferably having dimensions in the range of 2 mm to 15mm, more preferably 3 to 7 mm in length. The projections preferably havea caliper of 0.3 mm to 5 mm, more preferably 0.5 mm to 3 mm.

A first end of the projection is preferably attached to the body,preferably the intermediate and/or the central lumen, while a second endpreferably comprises one or more sensors. The second end is preferablyfree, ie, not attached to any of the lumen, and is adapted to beradially movable away from the central lumen.

The projections utilised in the present invention preferably comprisesensors, preferably temperature sensors.

One or more sensors, preferably 2 to 10 sensors, more preferably 2 to 6sensors may be utilised in the present invention. Preferably, eachsensor is mounted on a separate projection. In a particularly preferredexample, four projections, each having a single sensor mounted thereon,are provided.

Where more than one projection is provided, each projection ispreferably independently biased. Thus, each projection can follow thevessel morphology independent of the other projections.

The sensors are preferably located on an outer face of the projection,relative the central lumen, ie., facing the vascular tissue in use. Eachsensor should preferably be located toward, or at the distal tip of theprojection.

Where the detector is a variable capacitor, the capacitor plate(s)is/are preferably located on the inner face of the projection, relativeto the central lumen. In a particularly preferred example, fourprojections are provided, and each comprising a capacitor plate.

Where the detector is an inductance coil, the inductance coil(s) or themagnet(s) is/are preferably located on the inner face of the projection,relative to the central lumen.

The projections need not be mounted in substantially the samecircumferential plane of the catheter body, but this configuration ispreferred.

It is also possible to provide projections having different lengths.This allows a better assessment of the 3D location of the projections tobe provided while using a 2D imaging technique.

The projection preferably comprises a superelastic material.Superelasticity refers to the ability of certain metals to undergo largeelastic deformation. Such compounds favorably exhibit features such asbiocompatibility, kink resistance, constancy of stress, physiologicalcompatibility, shape-memory deployment, dynamic interference, andfatigue resistance.

A large number of super-elastic materials may be utilised, particularlybinary Ni—Ti with between 50 and 60 atomic percent nickel. While manymetals exhibit superelastic effects, Ni—Ti-based alloys appear to bebest suited for deployment in the human body due to them beingchemically and biologically compatible.

Preferably, the projection, when not restrained will adopt a deployedconfiguration in which a free end of the projection is extended awayfrom the central lumen. In this deployed configuration, the projectionis resiliently biased against the vascular wall in use, thus initiatingcontact between the sensor and said wall. This achieves an adequatecontact with the vascular wall, without substantially compromising bloodflow.

Preferably, a heat shrink wrapping is applied over at least a portion ofthe length of the projection. A heat shrink material is generally apolymeric material capable of being reduced in size upon application ofheat. These are generally used in the form of a tube. Suitable materialsinclude polyesters, PVC, polyolefins, PTFE and the like. The preferredmaterial is a polyester.

Preferably, the heat shrink material covers the detector and isolates itfrom the body of the subject of the interventional surgery. Preferably,the heat shrink material additionally covers the sensor.

In accordance with a particularly preferred example of the first aspectof the invention, the resiliently biased, projection when restrained,adopts a substantially straight shape, which lies substantially parallelto the longitudinal axis of the catheter body. In the deployedconfiguration, the projection adopts an arcuate shape along at leastpart of its length. In this embodiment, the gradient of the arcuateportion of the projection, with respect to the longitudinal axis of thecatheter, increases as a function of distance along the projection fromthe end attached to the catheter body. Thus, the free end of theprojection bends away from the catheter body. This particular embodimentallows the free end of the projections to more accurately andconsistently follow the morphology of the vascular tissue. A stenosisusually involves a section of the wall being inflamed and thusprotruding into the lumen of the blood vessel. Alternatively, acalcified plaque may have an irregular surface leading to it protrudinginto the lumen. Where an arcuate deployed projection is employed, thearc allows the tip of the projection to “reach around” to the trailingedge of a stenosed region as the catheter is moved along the vasculartissue. The arcuate nature of the projections also allows thetemperature sensors, where present, to be located more directly and incloser contact to the vessel wall as well as providing a more accuratemorphological resolution of the vessel wall. The maximum gradient of theprojection, with respect to the longitudinal axis of the catheter bodyis preferably less than 90°, more preferably less than 75°, morepreferably less than 60°. In this particular embodiment, the arc of theprojection preferably provides maximum possible contact angle betweenthe projection and the vessel wall of less than 90°, more preferablyless than 75°, more preferably less than 60°. This angle, while having amaximum deviation of less than 90°, is variable as a consequence of thecompliant nature of the biased projection. This allows the projection tofollow the vascular morphology.

Where a projection having an arcuate portion is provided, there may alsobe substantially straight portions of the projection along its length. Aparticularly preferred example provides a resiliently biased projectionhaving a substantially straight portion which bears the detector, inparticular, a capacitance plate. It would be advantageous to producecapacitance plates which remain substantially parallel even duringdisplacement of the resiliently biased projections. This may be achievedusing a sinusoidal shaped projection, preferably a flattened sinusoidalshape having about 1.5 wavelengths of a sinusoidal wave. This structurecould be described as an “extended S” shape. An example is shown in FIG.3. This shape provides an arcuate portion which enables good contactwith the vessel wall, as described above. It also provides a sectionwhich is flattened (in this case towards the middle of the projection)upon which the detector may be mounted.

In a particularly preferred example, the capacitor comprises two or moreresiliently biased projections, each of which may comprise one plate ofthe same variable capacitor. Thus, a preferred example is a cathetercomprising two resiliently biased arms, each of which comprise one plateof the same variable capacitor. Consequently, the resiliently biasedprojections may be positioned on opposed sides of the catheter at, forexample, 180° intervals. This allows the resiliently biased projectionsto bend away from the capacitor body and to contact opposed vascularwalls. As the resiliently biased projections are conformal to themorphology of the vascular tissue, a cross-section, or a series ofcross-sections of vascular morphology may be measured by relating thechange in capacitance to the change in distance between the plates ofthe variable capacitor as the projections contact the vascular wall. Itshould be understood that any number of capacitor plate pairs may beused in the present invention. These may each be mounted onindependently biased projections. Where, for example, 4 plates areprovided on 4 projections, the projections may be positioned atsubstantially 90° intervals. The opposed (180° separation) platespreferably bear paired capacitor plates. This allows an eccentricassessment of vessel dimensions rather than just concentric.

In another particularly preferred example of the first aspect of thepresent invention, a catheter is provided comprising two or moreresiliently biased projections each comprising one plate of differentvariable capacitors, and at least one fixed plate comprising the otherhalf of at least one of the variable capacitor plates on a resilientlybiased projection. This example allows the fixed plate to be provided,preferably mounted on, or integral with, the catheter body. The fixedplate is associated with at least one of the plates mounted on one ofthe resiliently biased projections. Thus, the capacitance can bemeasured between the projection-mounted plate and the fixed plate. Theterm “fixed” is intended to distinguish from the projections which aremoveable relative to the catheter body. “Fixed” is not intended to implythat the plate is stationary when in the vessel. In this particularexample, the same fixed plate or another fixed plate may be associatedwith the variable capacitor plate mounted on the second projection.

In an alternative example, the projection may be mounted to achieve asimilar resiliently biased effect. For example, one method of achievingthis would be to mount the projection on a spring, preferably amicro-spring, such that when unrestrained, the projection is extendedagainst the vascular wall as discussed above.

In an alternative example of the present invention, where inductance isused to assess the vascular dimensions, the device preferably comprisesa magnet on one of the resiliently biased projections, and a coillocated at another point which is capable of movement relative to theprojection. For example, the coil is preferably provided on the body ofthe catheter or another resiliently biased projection. The coil ispositioned such that the magnet lies in the axial plane of the coil. Asthe distance between magnet and coil changes, for example, the vesselnarrows and the projections bearing the coil and magnet move closertogether, the magnet will move closer to the coil. According toFaraday's law, any change in the magnetic environment of a coil willcause a voltage to be “induced” in the coil. The voltage can becalculated as follows:Voltage=−(number of turns)×(change of magnetic flux)/(time of movement).

It should be noted that:

-   -   (a) Voltage exists only when there is change of magnetic flux,        and    -   (b) the (−) sign expresses Lenz's law and shows that the coil        reacts against the external change of flux. Now, the magnetic        flux (F) in a single turn of the coil can be calculated as        follows:    -   F=(B)×(A)×cos[a]. This makes the Faraday formula:        Voltage=−n×D(B×A×cos [a])/Dt.

Where n=number of coil turns, B=magnetic field strength (in Tesla),A=area of the coil loop, a=angle between coil and magnet.

Since the A and a are stable in our case, we have:Voltage=−n×A×cos [a]×(DB/Dt)

In this formula, in the above circumstances, everything is substantiallyconstant except for the Voltage and the DB/Dt Thus, by measuring theinduced voltage in the coil, one can calculate theDB/Dt=−Voltage/(n×A×cos[a]).

DB/Dt is directly proportional to DS/Dt (where S is the distance thesensors moves towards each other), and this is the arterial wallacceleration. The arterial wall acceleration is an indicator of arterialwall elasticity.

For example, if the coil loop diameter is 1 mm, there are 100 loops andthe magnet is axial to the coil (angle a=0). In this configuration letsay that as the coronary artery moves, we measure a voltage 1 mV at theends of the coil, that existed for 1 sec.

The area of the loop will be 78.5×10⁻⁸ m². The DB/Dt will beDB/Dt=−0.001/(100×78.5×10⁻⁸×cos(0))=12.7 Tesla/sec.

From the arterial wall acceleration, one can calculate the distance thesensors traveled if we measure the time Dt that the sensors moved. Thisis equal to the duration time of the voltage, since there is onlyvoltage when there is relative movement between the coil and the magnet.

The sensors may be any form of temperature sensor and are preferablyselected from thermistors, thermocouples, infra red sensors and thelike. Preferably, the sensors are thermistors. These are preferablysemi-conductor materials having an electrical impedance in the range of1-50 KΩ. Such thermistors prove extremely reliable regarding therelation between the temperature changes and resistance changes.

Preferably, the catheter comprises a radiopaque marker which aids in thelocation of the device by fluoroscopy during interventional surgery.More preferably, at least one detector includes a marker so that it isdiscernible via fluoroscopy. Most preferably, individual detectorsinclude different marker types, so that using fluoroscopy, theindividual detectors can be identified and their spatial orientation andrelative location to a desired part of the vessel wall thus clearlydefined.

The detector is preferably attached to an electrical carrier, preferablya wire, which allows the data from the detector to be connected to aremote device to the personal computer. The wire(s) are preferablyhoused within the sheath and are preferably electrically isolated fromthe patient. Preferably, the wire(s) are housed between the centrallumen and the intermediate lumen, within the outer sheath.

The proximal section of the catheter incorporates a connector forcoupling the detected signals to a remote device such as a personalcomputer. These signals are transmitted along the wire(s) from thedetector. The wire(s) are preferably housed within the sheath and arepreferably electrically isolated from the patient. Preferably, thewire(s) are housed between the central lumen and the intermediate lumen,within the outer sheath.

Where sensors are provided, these may be similarly linked via anelectrical carrier to a remote device.

In a particularly preferred example, electrical carrier connected to thedetector and/or sensor for transmitting data to a remote device iscoiled. Preferably, the electrical carrier is coiled around the body ofthe projection. Such a device is described in our earlier filed Europeanpatent application no. 01306599.0 In this embodiment, the electricalconnection is coiled to reduce the strain at critical points where it isnecessary to maintain a seal, and hence electrical isolation. The coilednature of the carrier also allows the carrier to act as an inductancecoil.

The design is also especially suitable for use with a vascularthermography catheter apparatus of the type described in our earlierfiled International patent application no. PCT/EP01/04401.

In a particularly preferred embodiment of the present invention, thecatheter may be used in concert with a catheter positioning system inaccordance with that disclosed in our co-pending European applicationNo. 01307682.3. This system comprises a guide catheter extension adaptedto co-operate with a guide catheter, a catheter positioning deviceadapted to engage a catheter and guide the catheter within the guidecatheter extension, wherein the guide catheter extension furthercomprises a plurality of engagement means for fixing the relativepositions of the guide catheter extension and the positioning device atany one of a number of positions over its length.

This system allows the distance between a guide catheter and apositioning device to be manipulated by the user. Thus the guidecatheter may be fixed in position relative to both patient andpositioning device, while providing the optimum distance between theeffective length of the guide catheter (guide catheter and guidecatheter extension) and the points at which the catheter is fixed to thepositioning device.

The guide catheter extension is adapted to receive a catheter used ininterventional cardiology. Preferably, the body of the guide catheterextension is substantially cylindrical in cross section and has adiameter in the range of 1-15 mm. Preferably the diameter is in therange of 2-10 mm, more preferably 3-7 mm. Preferably, the length of theguide catheter extension is in the range of 0.1 m to 1 m.

More preferably, the length of the guide catheter extension is 0.15-0.5m.

The body of the guide catheter extension may be formed from standardguide catheter materials. For example nylon, PTFE, polyurethane,polyethylene and nitnol and mixtures thereof may be used. It may also bemade from metals such as aluminum, steel and alloys thereof.

The guide catheter extension preferably has a number of points adaptedfor engagement with the catheter positioning device. Notches, annularindentations, and any other suitable means may be used. Preferably thereare 2-200 fixation points, more preferably 5-100, most preferably 10-50fixation points. These engagement means enable the guide catheterextension to be fixed in place, at selected positions over its length,on the catheter positioning device.

The guide catheter extension comprises a distal and a proximal end.Preferably, the distal end is adapted for engagement with the guidecatheter, while the proximal end is adapted for engagement with thecatheter positioning device.

There is also provided a guide catheter extension capable of receiving acatheter, comprising a substantially rigid tubular section capable ofsealing engagement within a compression fitting of the guide catheter.

Where positioning of the catheter, therefore translational movementwithin the vascular tissue (therefore also within the guide catheter andguide catheter extension) is required, the arrangement allows thejunction between the guide catheter extension and guide catheter to besealed by tightening the compression fitting, but does not allow thejunction to impinge on the catheter within. The seal is preferablyachieved by providing a sealing element in the guide catheter extensionwhich forms a low friction, slidable seal with the sheath of thecatheter. Thus the catheter is able to be moved and positioned withinthe apparatus without undue friction being applied to the catheter. Thisis particularly important as a Y-piece, in addition to being used as theinjection point for contrast medium into the patient, is also used as apressure measurement point during the interventional procedure. In orderfor the pressure of the patient to be reliably measured, the system mustbe substantially closed, otherwise the pressure will vent at a nonclosed section. This will lead to loss of pressure, loss of blood, andunreliable pressure readings. However, the present system maintains thepressure of the system as the guide catheter and guide catheterextension junction is sealed and the diameter of the catheter isgenerally slightly less than the diameter of the internal lumen of theguide catheter extension. Alternatively, the pressure is maintained byproviding the above mentioned sealing element in the guide catheterwhich forms a low friction, slidable seal with the sheath of thecatheter.

Most preferably, the distal end of the guide catheter extension isadapted for engagement with a standard Y-piece used in interventionalcardiology, having a compression fitting. This substantially preventsloss of blood or fluid at the junction between the guide catheter andthe guide catheter extension.

Preferably the distal end of the guide catheter extension comprises asubstantially rigid tubular section which is fixed to a flexiblesection, and which is co-axial therewith. The rigid tubular section maybe integrally moulded with the flexible section. Alternatively, it isfixed to the flexible section by any suitable means, for example, glue,soldering, welding and the like.

The catheter positioning device is preferably a type for positioning acatheter and comprises a first lumen mount for holding a first lumen ofthe catheter, a second lumen mount for holding the guide catheterextension, and a drive mechanism, wherein the first lumen mount isselectively connectable to the drive mechanism for relative movementwith respect to the second lumen mount.

The second lumen mount preferably includes a bracket, preferably adaptedfor engagement with the guide catheter extension. The bracket is usuallylocated at one end of an extension arm, while the other end is connectedto the body of the positioning device.

The positioning device is preferably a pull-back device which isparticularly useful when used in concert with a vascular catheterapparatus according to a first aspect of the present invention. Thecatheter requires precise positioning and or maneuvering within vasculartissue. The positioning device also allows precise and controllablemovement within the vascular tissue. This enables the precise vascularmapping of the vessels morphology.

When the pull-back device is used in concert with the types of vascularcatheter according to the present invention, the pull-back devicepreferably comprises a first lumen mount for holding a first lumen ofthe catheter, and a second lumen mount for holding a second lumen of thecatheter, a third lumen mount for holding a guide catheter extension,and a drive mechanism, wherein each of the first and second lumen mountsis selectively connectable to the drive mechanism for both independentand relative movement with respect to the third lumen mount and to oneanother to control the configuration of the catheter.

The pull-back device enables a guide catheter and the catheter to bestabily mounted. In particular, the pull-back device enables relativemovement between the guide catheter and the catheter but, in use, allowsthe catheter to move relative to the patient and restrains movement ofthe guide catheter relative to the patient. The pull-back deviceadditionally allows a controlled retraction and positional retention ofthe associated sheath, thus ensuring atraumatic expansion of theprojections on the catheter.

Preferably, the pull-back device comprises a fixed mount for the guidecatheter extension, a mount for the sheath and a mount for the combinedinner and intermediate lumen. Hereinafter, the guiding catheterextension mount is referred to as mount A, the sheath mount as mount B,and the inner and intermediate lumen mount as mount C.

Mount A preferably has a fixed position during pull-back but may beadjustable. Mount B and C are preferably moveable relative to oneanother and to mount A. Mount B and C may be motor driven, in particularstepper motor driven. While mount B and C are moveable, they arepreferably adapted to enable selective locking in place relative to oneanother and/or to mount A. Mount C is preferably mounted on the drivemechanism although mount B and C may both be mounted on the drivemechanism. The drive mechanism enables the catheter to be driven towardsor away from the patient via movement of mounts B and/or C.

The interlocking of mount B and C prevents the sheath from movingrelative to the lumens housed inside the sheath, thereby ensuring theprojections remain in the deployed configuration and engaged with thevascular tissue in the area of interest.

The locking mechanism on the pull-back device includes a restrainingmechanism, preferably a stopper rod. This is provided with means forengaging projections within mounts B and/or C. A similar set ofprojections within the same mounts are used to selectively connect themounts to the drive rod. These projections may be actuated by a user whocan selectively control which of the mounts is locked and which aredriven, and the interaction between the mounts.

The drive mechanism is preferably driven by a motor, and preferablygearing is provided along with control and monitoring means.

It is particularly important that substantial occlusion of the vasculartissue is prevented. This is achieved by the present invention as theapparatus in a deployed configuration does not substantially increaseits radial cross sectional area beyond the radial cross sectional areaof the apparatus in a retracted configuration.

Preferably, the ratio of the area of the cross-sectional profiles of theapparatus in the deployed to retracted configurations is in the range4:1-1:1, preferably 3:1-1.25:1, more preferably 2.5:1-2:1, mostpreferably 1.75:1-1.25:1.

The vascular catheter apparatus of the present invention, subsequent tothe identification and measurement of vascular tissue, in particular,atherosclerotic plaque, may be used to treat an area identified as beingat risk of rupture of said plaque. Treatment may be effected byreinserting the catheter to a predetermined area of the vascular tissue.This reinsertion may be achieved in a controlled manner as the priormorphology measurement scan with the device may be used to produce amorphological map of the vascular tissue. This information may be storedin the remote device and can be used to relocate the area of risk. Thisprocedure requires less contrast media to be infused into the patientthan would normally be required in similar vascular interventionalprocedures as the position of the catheter is known due to the datastored in the remote device. The pull-back device may then, under thecontrol of a user, be used to drive the catheter back to, for example,the starting point of the morphological measurement or any point alongthe path of the morphological data acquisition, for furthermorphological or physiological measurements or alternative treatments ofthe vascular tissue.

For example, the catheter apparatus can then be used to treat the areaby any of the usual therapeutic procedures, including localised deliveryof a therapeutic agent, delivery of a stent, brachy therapy, ablation ofselected tissue etc. Thus the catheter may additionally compriseangioplasty balloons or sleeves.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of the present invention will now be described in detail withreference to the accompanying drawings, in which:

FIG. 1 shows a schematic diagram of a system for conducting vascularcatheterisation of a patient;

FIGS. 2 and 2 a shows a side view of the distal end of the catheter ofthe present invention;

FIG. 3 shows a sectioned view of the catheter of the present inventionwith an extended S-shape profiled projection;

FIG. 4 shows the pull-back device in side view;

FIG. 5 shows the pull-back device in plan view;

FIG. 6 shows a cross-sectional view of the catheter guide extension;

FIG. 7 is a flow diagram illustrating the steps involved with conductingintravascular catheterisation of a patient and the associated datacapture and image processing; and,

FIG. 8 shows an example of a signal processing unit for use with acatheter of the present invention.

DETAILED DESCRIPTION

FIG. 1 is a schematic diagram of a system for conducting vascularcatheterisation of a patient.

The system includes a personal computer (PC) 1 that presents a generaluser interface (GUI) via a number of monitors 2. The user interfacesystem is based on a Microsoft Windows™ platform. Multiple windows maybe used to acquire/project data from/to the user. Although not shown,the PC can accept user inputs via a keyboard and mouse, or otherpointing device, in the usual manner. The PC includes a number of datastores 7, which may be external, and a CD ROM reader/writer device 3.

The PC is coupled via a data interface 4 to a catheter 5, details ofwhich will be described below. In this example, the catheter 5 transmitsfour channels (one for each detector) which are received by the datainterface 4. An analogue capacitance data signal on each channel isconverted to a digital signal using an A/D converter within the datainterface 4 at a user configured sampling rate of up to 2.5 KHz.Typically, the sampling rate would be set at around 25 to 50 Hz toreduce the quantity of data acquired.

The data interface 4 includes a multiplexer (not shown) that combinesthe four digital channels into a single time division multiplexed (TDM)signal. This TDM signal is coupled to the PC over a PCI bus. The datafrom each channel is written into an area of memory within the datastore 7 reserved for that channel where it can subsequently be retrievedfor data processing along with the corresponding time sequenced datafrom other channels and image data from other sources.

The capacitance data from the catheter 5 is introduced to the systemsoftware running on the PC using function calls. Capacitance data areinput to the software as the frequency at the A/D hardware inputs, andtherefore they have to be converted to distance. The frequency changesare first converted to voltage via a frequency to Voltage converter, andthen they are driven to the A/D coverter. A detector data convertfunction handles this process.

This particular system is designed to be used in conjunction withtemperature sensing apparatus. The temperature data can be processed ina similar way to the capacitance data, as discussed in the precedingparagraphs.

The system is designed to be used in conjunction with a fluoroscopyx-ray apparatus and therefore includes a video frame capture interface 6that couples fluoroscopy video data inputs to the PC via a PCI bus.Similarly, it can be used in conjunction with intravascular ultra-sound(IVUS) image data fed from the catheter 5 (when provided with theappropriate hardware). The system software allocates sufficient memoryarea to the systems memory for this data, taking into account thecurrent system configuration, for example sampling rate, recording time,and video frame size. A memory handle hDib is used to map video datadirectly through the PCI bus from the video frame capture interface 6 tothis allocated area in memory. hDib memory is divided into i equalchunks, each of a size equal to the frame capture interfaceframe-buffer. Optionally, hDib [i] data can also be mapped to a memoryarea of a screen-video buffer, giving capability of live preview duringrecording. Each time the software records an x group of four (or more)capacitance measurements, it prompts for a frame capture at hDib [x]. Auser configuration file determines the ratio between capacitancedata:fluoroscopy video frame capture.

Whilst in normal circumstances the catheter 5 is inserted manually, itis intended that when performing vascular measurements the catheter 5 ispulled back relative to a predetermined start position using anelectromechanical pull-back drive 8 coupled to the body of the catheter.The pull-back drive 8 is controlled by the PC via a pull-back driveinterface 9. The system software accesses user-defined configurationfiles to get the necessary information about controlling the systemsautomatic pull-back interface 9. Data sampling rate, recording durationand pre-selected retraction rate are taken into consideration foradjusting the pull-back speed. The software routines control a D/Aconverter (not shown) that feeds the input of the pull-back interface 9with an appropriate control voltage. The controlled pull-back processwill be described in more detail below.

Capacitance data plotting may be both on-line and/or off-line. In anon-line mode, the monitor presents a capacitance/time-distance graph,where capacitance is continuously plotted as connected dots. In anoff-line mode, capacitance data can be loaded from the data store 7 (orother media) and plotted on the screen graph. The user can scroll todifferent time/temperature locations, while several automated functionsmay be provided.

The system software is designed to provide basic and advanced imageprocessing functions for the captured fluoroscopy/IVUS video frames,such as filtering and on-screen measurement functions. The user canfilter the captured frame to discard unwanted information while focusingon the desired one. There are several auto-filter options as well asmanual adjustment of the image curve. In addition, the user cancalibrate the system and proceed in performing on-screen measurements ofboth distances and/or areas. Automatic routines perform quantificationof the measurements giving significant information on lesioncharacteristics.

By using capacitance data and video frame data, the system software usesadvanced algorithms based on interpolation and fractal theory to plot a3D reconstruction of the vessel under measurement. The user can freelymove the virtual camera inside the reconstructed vessel in 360°, and/orfly-through the vessel. 2D reconstructions are also provided.

FIGS. 2 and 2 a shows one example of the distal tip of a catheterincorporating sensors 10 mounted circumferentially about a central lumen14. In this example, four sensors 10 are mounted on resiliently biasedprojections 11 circumferentially about the central lumen at 90°intervals, although only two sensors are shown here for the sake ofclarity.

Variable capacitor plates 12 and 12 a are mounted on the side of theface of the projections facing the central lumen 14. In this example,four variable capacitor plates 12 and 12 a are mounted on resilientlybiased projections 11 circumferentially about the central lumen at 90°intervals, although only two variable capacitor plates, 12 and 12 a, areshown here for the sake of clarity.

In this example, the opposed plates, 12 and 12 a, are a pair of platesmaking a single variable capacitor.

Each plate 12 and 12 a is embedded within a plastics covering, althoughit could instead be surface mounted. The shape and configuration can bemodified to provide different shaped plates, different plate spacings,and different longitudinal coverage for the or each pair of plates.

Each plate 12 is connected to the proximal part of a catheter (notshown) via a respective thin electrical wire 13 carried within the bodyof the catheter 10 (in the Figure, some electrical wires have beenomitted for clarity). Each electrical wire 13 is electrically shieldedalong its length to avoid interference. As will be described in detailbelow, each electrical wire 13 connects to an interface forming part ofa signal processing system that is used to detect changes in theeffective capacitance presented by each pair of plates 12 and 12 a. Asan alternative, portions of the signal processing system described belowcan be incorporated within the body of the catheter itself to eliminateinterference.

The sensors 10 are NTC thermistors. Such thermistors prove extremelyreliable regarding the relation between the temperature changes andresistance changes. An NTC thermistor having a 30 KΩ impedance at 25° C.typically maintains linearity between 35° C. and 45° C., at a resolutionof 0.01° C.-0.1° C.

The construction of the thermistors 10 are that of two rectangularplates with a metal alloy oxide in the centre. The thermistor hasdimensions in the range of 0.25 mm-5 mm, and a caliper less than 1 mm.

Each thermistor 10 is permanently attached to the end of each projection11 by bonding with an thermally conducting epoxy glue 16. Eachthermistor 10 is permanently connected to an insulated wire 17,preferably an insulated bifilar wire.

The wire 17 has a low impedance and is constructed from nickel and/orcopper. This wire provides an electrical connection with the proximalend of the device (not shown). The projections 11 are mounted on thecentral lumen 14 and sandwiched between the central lumen 14 and anintermediate lumen 18. The point at which the projections 11 meet thecentral/intermediate lumen terminus is sealed. This means that thecomponents located between the central and intermediate lumen areelectrically isolated from the patient except through the projections.This also means that no air or debris which may find its way into thespace between the lumen can be transmitted to the patient.

As shown in FIGS. 2 and 2 a, the catheter is mounted on an angioplastyguide 19 wire which runs through the central lumen 14 and a guide member20 which defines the distal tip of the catheter.

In use, the apparatus may be actuated between a non-wall-temperaturesensing configuration and a temperature sensing configuration. Thenon-temperature sensing configuration is hereinafter referred to as theretracted configuration. The temperature sensing configuration ishereinafter referred to as the deployed configuration. An example of thedeployed configuration is shown in FIG. 2. An example of the retractedconfiguration is shown in FIG. 2 a.

In the retracted configuration, a sheath 21 encompasses the projections11 so that they are constrained to lie parallel to the longitudinal axisof the catheter and therefore cannot take up a deployed position. Thesheath 21 extends as far as the rear end of the guide member 20 but doesnot overlap the guide member. This minimises any protrusions from thecatheter which could lead to damage of the vascular wall. This isparticularly important where a vessel is angulated or there isbifurcation of the vessel. Such features lead to bending of the catheterand would emphasize any protrusions. Hence, in this example the sheath21 and the guide member 20 present a smooth profile when adjacent to oneanother in the retracted configuration.

To adopt the deployed configuration, the sheath 21 is withdrawn awayfrom the extreme distal tip i.e., away from the guide member 20, towardsthe proximal section, to expose the projections 11. When the sheath 21is withdrawn to the extent shown in FIG. 2, the resiliently biasedprojections 11 take up the deployed configuration. It should be notedthat the sheath is controlled from the proximal end of the apparatus andis not shown in its entirety in the Figures.

In the deployed configuration, the sheath 21 is retracted until it is atleast level with the mountings for the projections 11 on theintermediate lumen 18 so that it does not impede the movement of theprojections.

The projections are made of NiTinol and take on the deployedconfiguration automatically due to their superelastic properties.

It should be noted that each projection 11 is effectively independentand thus may extend to the vascular wall in the deployed configurationbut will not exert high levels of force upon the wall.

An excessive force should not be exerted on the vascular wall. This willvary between one type of vascular wall and another. The apparatus shouldexert enough force to enable an adequate thermal contact between thesensors 10 and the vascular wall. More particularly, when the catheteris in the deployed configuration, preferably all of the projections 11are in contact with the vessel wall at any one point in time.

The projections 11 individually extend a certain angle of expansion (r)away from the longitudinal axis of the catheter. In the deployedconfiguration, r has a value in the range of 15°-70°. However, r is notfixed and varies with the diameter of the vascular tissue being measureddue to the flexibility of the projections 11.

Different diameter catheters may be used for different diameters ofvascular tissue. However, as it is desirable to minimize the diameter ofcatheters in all interventional vascular treatments, it is desirable toadapt the length of the projections and/or the angle to which theprojections may extend away from the central lumen depending on thedimensions of the vascular tissue being measured rather than increasingcatheter body dimensions. Thus, the projections for a large bloodvessel, for example 8 mm diameter, will generally require a length ofprojection in the range of 5 mm to 10 mm. Smaller diameter vasculartissue, for example 2.5 mm diameter, will generally require a length ofprojection in the range of 2 mm to 6 mm. Typically, the ratio of thearea of the cross-sectional profiles of the apparatus in the deployed toretracted configurations is up to 4:1.

The catheter includes a valve system (not shown) allowing the centrallumen 14 to be flushed in an adequate way, thus minimising thepossibility of air bubbles or debris within the lumen. Such a valve isconstructed to enable engagement by a 2 mm, 5 mm, or 10 mm, 60 luersyringe. The catheter may be flushed with a suitable fluid such assaline. When flushing the catheter, fluid should exit via the distal tipof the catheter, indicating proper flushing of the central lumen 14. Theproximal section of the catheter (not shown) incorporates a connectorfor the capacitance and temperature signal transfer to the datainterface 4. The connector contains five female plugs to assure propertransmittance of the electrical voltage signals transmitted from thefour thermistors 10, and the frequency signals transmitted from the fourcapacitor plates 12. These signals are transmitted along the wires 17from the four thermistors 10 and the four wires 13 from the 4 capacitorplates 12. The five female plugs concerned with plates 12 are connectedto four detector wires and one common ground. A directional, 5 pin, goldplated, water-resistant connector is used.

FIG. 3 shows the deployed configuration projection adopting an arcuateshape along part of its length, with the gradient of the projection,with respect to the longitudinal axis of the catheter, increasing as afunction of distance along the projection from the end attached to thecatheter body. The projection shown adopts an “extended S” shape. Asdiscussed above, this allows the arcuate portion, at the distal end ofthe projection, to achieve adequate contact with the vessel wall, whileproviding a section, towards the middle of the projection, where thecapacitor plate is mounted. This section remains relatively parallel tolongitudinal axis of the catheter body, even upon radial displacement ofthe projection.

As shown in the FIG. 3, the wire 17 is coiled around the length of theprojection 11. This feature has the effect of substantially eliminatingstrain when the projection 11 flexes. The pitch of the coil is typicallyarranged to be such that there are 5 to 10 turns over a length of 10 mm.As will be described below, a heat shrink wrapping 22 is applied overthe projection 11 to prevent damage to the wire 17 during retraction andreplacement of an outer sheath 21. The heat shrink wrapping alsoprovides an additional degree of electrical isolation.

To assemble a projection, a NiTinol arm is first pretreated by placingit in a bending tool and heating to around 700° C. to impart a bend(s)in the arm. The NiTinol arm is then held straight in a chuck and athermistor/bifilar wire assembly is attached to a free end of the armusing a UV cure adhesive. The wire 17 is then spun around the length ofthe NiTinol arm. Finally, the heat shrink wrapping 22 is placed over thelength of the NiTinol arm to a point just beyond that of the thermistor.In this example, the heat shrink wrapping is supplied as a polyestertube that is cut to length. An epoxy resin is then injected into the endof the tube. The assembly is subsequently heat treated to shrink thetube and set the epoxy resin. The heat shrink wrapping is then trimmedback to expose at least part of the epoxy resin coated thermistor, whilemaintaining electrical isolation of the bifilar wires. After heattreatment, the heat shrink has a wall thickness of around 10 μm. Thecapacitor plate may be attached to the projections prior toencapsulation, or may be attached to the outside of the shrink wrappingand further encapsulated with another shrink wrapping.

The body of a pull-back device is illustrated in FIGS. 4 and 5. Theproximal section of the catheter described above is constructed toenable remote deployment and retraction of the projections. This iseffected via manipulation of the sheath. A two-lumen telescopicconstruction 23 is used to manipulate the sheath 21 between theretracted and the deployed configuration. One lumen is connected to, orintegral with, the outer sheath and can slide over an adjacent lumenwhich comprises or is connected to one of the lumen housed within thesheath. Rotation of one tube inside the other is prevented by slottingof the lumen or other features on the lumen. Additionally, scalingmarkings (not shown), may be provided to avoid over-retraction of thetubes.

The pull-back device includes a drive module 24 which includes a motor,gearing system, typically a speed reducer, control and monitoring means,and engagement gear for a driving rod 25. The drive module may be formedseparately from the body of the pull-back device so that it may bereused. The body of the pull-back device must be kept sterile and may beformed from a material such as polyurethane. This allows the body to becheaply and easily produced and may be disposable. Alternatively, oradditionally, the pull-back device may be enclosed in a sterile,flexible plastic sheath when in use, so as to maintain sterility.

The pull-back device comprises a driving rod 25, adapted for engagementwith an engagement gear of the drive module 24 and mount C. Mounts C andB are adapted to engage the central/intermediate lumen 26 and the sheathlumen 21 respectively. A Mount A is provided which is adapted to engagethe guide catheter extension 27. Mount A includes a bracket 28 forconnection of mount A to the guide catheter extension fixation points29. When engaged, mount B may be moved towards C to place the catheterin the open configuration. C may be selectively driven reversibly over arange of travel (usually about 60 mm) suitable for withdrawal of thecatheter apparatus over the measured region. The driving rod 25 is aworm-screw type which interacts with the engagement gear of the drivemodule 24, thus providing a smoothly driven apparatus.

The mounts B and C may individually be locked in position relative toone another or may be selectively unlocked in order to allow movement ofthe lumen 26, sheath 21 and guide catheter 27 relative to one another.

With reference to FIGS. 6 and 7, in use, the sequence of events beginswith the insertion of a guiding catheter into the area of generalinterest (step 100), for example the cardiac region. Where, for example,the coronary arteries are to be examined, the guiding catheter isinserted so that it is adjacent the opening of the coronary arteries(step 110). An angioplasty guide wire is then inserted into the coronaryartery, past the point of specific interest. The guide wire is usuallyinserted with the aid of standard fluoroscopic techniques, as is theguide catheter.

The guide catheter, when in place over the entrance to the coronary (orother target) artery will protrude a distance from the patient once inplace. This is then fixed to the guide catheter extension 27. The guidecatheter extension will be fixed to the guide catheter by inserting thenon-compressible tube 30 into the Y-piece 31. The gland nut 32 ando-ring seal (compression fitting) is tightened to seal the joint betweenthe guide catheter and guide catheter extension and a securing means 33is provided which holds the Y-piece in place relative to the guidecatheter extension. Alternatively, the outside surface of thenon-compressible tube may be profiled with shallow circumferentialgrooves, to ensure that the tube will not pull out when held in thecompression fitting of the Y-piece (not shown).

A seal element 34 is provided within the guide catheter extension. Thisis sandwiched between the non-compressible tube and the guide catheterextension body. This provides a sealing engagement between thenobn-compressible tube and the catheter.

Once the guide catheter, guide catheter extension and guide wire are inposition, the catheter 5 of the present invention is maneuvered over theguide wire to a position beyond the specific area of interest in thecoronary artery (step 120) with the aid of fluoroscopy. The catheter isthen fixed in position on the pull-back device by clipping into mounts Band C. The guide catheter extension is then fixed in position on themount A, at a fixation point along its length which optimises thedistance between mount A and B and C. Thus, the guide catheter extensionshould be fixed to mount A so that the catheter may be mounted on mountsB and C in a closed configuration.

An angiogram is taken (step 130) to assess the position of the catheterin the vascular tissue. This image is saved and the position of thecatheter is marked on the image so as to define a starting point for thecontrolled pull-back step.

The sheath 21 is then be retracted to allow the projections to adopt thedeployed configuration. This is achieved by moving mount B towards mountC (usually manually). Mount C at this time is locked relative to mountA. Once the sheath 21 is retracted sufficiently to allow expansion ofthe resiliently biased projections, mount B is locked in position andmount C is pulled back by the drive mechanism until the projections arehoused in the sheath. This is feasible if the sheath 21 is retractedsufficiently (equal or greater than the length of the pull-back distanceduring which measurement takes place) to allow the intermediate/centrallumen 26 to be retracted in the sheath 21 without the sheath impactingon the projections along the length of measurement.

Alternatively, the mount B and C are locked in position once thecatheter is in the deployed configuration and both mounts are pulledback by the drive mechanism.

The locking mechanism includes a stopper rod 35. This is provided withgraduations capable of engaging electrically actuated locking pins (notshown) within mounts B and/or C. A similar set of electrically actuatedlocking pins (not shown) within the same mounts are used to selectivelyconnect the mounts to the drive rod 25. A set of locking pins on anyparticular mount may not be connected to both the drive rod 25 and thestopper rod 35 simultaneously. Thus, each mount is either in drive orstop mode. Alternatively a ratchet mechanism may be provided as thelocking mechanism.

When the mount C is in drive mode, it moves relative to mount A and B.Mount C cannot be moved towards mount B when attached to the pull-backdevice.

The catheter may be marked to indicate when the sensors are in adeployed or in a retracted position. This may be achieved by provisionof a telescopic tubing 23 with appropriate indicators or by simplymarking the extreme deployed or retracted position on the apparatus.

Controlled pull-back of the catheter then takes place (step 140). Thepull-back takes place at a constant speed and is controllable by theuser. Pull-back typically takes place at speeds of 0.1 to 2 mm indivisions of 0.1 mm or so.

The pull-back takes place over a distance of the vascular tissue beingmeasured. Capacitance and/or temperature readings may be takenintermittently or substantially continuously. The data transmitted bythe detectors from the vascular wall is captured for data and imageprocessing (step 150) together with a fluoroscopy/IVUS image frame.

As the catheter is withdrawn inside the artery, the projectionsautomatically adjust their angle following the wall's morphology withoutlosing the desired contact. The result is that the contact between theprojections and the wall is continuously maintained, even when thecatheter is crossing very irregular plaque formations.

Once the pull-back has been completed, the central/intermediate lumensare retracted such that the projections are withdrawn into the sheath 21in order to place the sensors and detectors in the retractedconfiguration. This restores the original smooth profile of thecatheter. The catheter may then be detached from the pull-back deviceand withdrawn from the patient or may be reinserted into the same oranother blood vessel in order to take another reading. Alternatively,the catheter may be reinserted in order to enable a therapeutic orsurgical intervention.

An example of a signal processing system 40 for use with the catheter 5is shown schematically in FIG. 8. Each signal channel includes avariable frequency oscillator 41 connected to a respective one of theplates 12 and 12 a at the distal tip of the catheter 5. When there is analteration of arterial wall morphology, ie a lesion effectivecapacitance between a plate 12 and the adjacent plate 12 a will vary,thereby changing the output frequency f₁ of the associated variablefrequency oscillator 41. The output f₁ of the variable frequencyoscillator 41 is fed to a mixer 42 where it is mixed with the outputfrequency f₂ of a fixed frequency oscillator 43 to produce sum (f₁+f₂)and difference (f₁−f₂) frequencies. The fixed frequency oscillator 43may be common to each channel. The sum frequencies are typicallyfiltered out to leave the difference frequencies, which are fed to amicroprocessor based signal processor 44 for analysis and subsequentdisplay 45. The difference frequencies are typically in the RF range of0-20 KHz.

The microprocessor based signal processor 44 incorporates software thatimplements a number of different forms of signal analysis. This mayinclude a spectrum analyser (not shown) which analyses each signalchannel and provides correlation between different channels. This datacan be used to generate views of the vessel wall to indicate morphologyand areas of compositional interest.

In operation, it is necessary to insert the catheter 5 and position itat a desired location. The system must then be calibrated so that thedifference frequency (f₁−f₂ is detected to be zero. This is achieved bytuning the output frequency f₂ of the fixed frequency oscillator 43 by asmall amount using an associated phase locked loop control mechanism(not shown). As indicated, this can be performed automatically using afeedback control loop 46. Once the system is correctly calibrated, acontrolled pullback (or insertion) of the catheter 5 can be initiated tobring it into the region of interest. Depending on the configuration ofthe array of metallic plates 12, data can be logged automatically whilstthe catheter 5 remains stationary, or alternatively the catheter 5 canbe moved continuously over a length of the vessel of interest.

1-14. (canceled)
 15. A catheter comprising at least one resilientlybiased projection and at least one detector which generates a signalwhich varies as a function of radial displacement of the at least oneresiliently biased projection relative to the longitudinal axis of thecatheter.
 16. A catheter according to claim 15, wherein the at least onedetector is a variable capacitor.
 17. A catheter according to claim 16,wherein one plate of the variable capacitor is mounted on the at leastone resiliently biased projection.
 18. A catheter according to claim 17,wherein the capacitor plate is located on the inner face of the at leastone projection, relative to the body of the catheter.
 19. A catheteraccording to claim 16, wherein one plate of the variable capacitor isformed integrally with the at least one resiliently biased projection.20. A catheter according to claim 19, wherein the capacitor plate islocated on the inner face of the at least one projection, relative tothe body of the catheter.
 21. A catheter according to claim 15, whereinthe at least one detector comprises an inductance coil and a magnet. 22.A catheter according to claim 21, wherein the inductance coil is mountedon the at least one resiliently biased projection.
 23. A catheteraccording to claim 21, wherein the inductance coil is integrally formedwith the at least one resiliently biased projection.
 24. A catheteraccording to claim 15, wherein each at least one projection isindependently biased.
 25. A catheter according to claim 15, wherein eachat least one detector is mounted on a separate projection.
 26. Acatheter according to claim 15, wherein the at least one projectioncomprises a superelastic material.
 27. A catheter according to claim 26,wherein the at least one projection comprises a nitinol.
 28. A catheteraccording to claim 15, wherein the at least one resiliently biasedprojection, when deployed, adopts an arcuate shape along at least partof its length.
 29. A catheter according to claim 15, additionallycomprising a signal processing system electrically coupled to the atleast one detector, which is adapted to detect changes in the signal ofthe at least one detector.
 30. A method of studying the physiologyand/or the morphology of a vessel wall comprising detecting capacitancevariations in detectors inserted into the vascular tissue.
 31. A methodof studying the physiology and/or the morphology of a vessel wallcomprising detecting inductance variations in detectors inserted intothe vascular tissue.